“Forest health” is an important concept often not covered in tree, forest, insect, or fungal ecology and biology. With minimal, inexpensive equipment, students can investigate and conduct their own forest health survey to assess the percentage of trees with natural or artificial wounds or stress. Insects and diseases in the forest are the focus, though student guides could be modified for many terrestrial or aquatic systems, depending on location. The lesson is geared toward older students, with suggestions for adaptation in earlier grades as well.
The health of our forest ecosystems is vitally important not only for the plants and animals that live in them, but also for our own ecological, economic, and aesthetic values. Forests produce oxygen and clean water, hold soil in place, and recycle nutrients. Tourism, recreation, timber, fiber, and hunting are just some of the multibillion-dollar industries that are dependent on forests each year. Forestry is an important aspect of biology that many students are exposed to, but few come away from the classroom with an understanding that forests and trees have diseases and stresses just as we do. Yet lessons that include forest health aspects such as insect attacks or destructive logging do not impart that damage and decay are essential components of a healthy, recycling forest.
This forest health lesson can be correlated in many ways with other forestry and biology curricula. Previous articles in ABT have integrated relevant forestry and ecology principles into biology lessons (Varbalow, 1963; McNew, 1966; Rhoades, 1974; Zipko, 1983; Glenn, 1995; Wells Rollinson, 2012). Project Learning Tree (http://www.plt.org), Project Wild (http://www.projectwild.org), and many agencies have dendrology (Glenn & Dickmann, 2009), entomology, or mycology lessons available, but what most do not incorporate well is the impact these disciplines have on overall forest health.
Recent trends and publications note the importance of hands-on nature learning and student engagement in learning in K–12 and above (Lightbody, 2008; Louv, 2008; Ruiz-Primo et al., 2011). Insects and fungi are easy to find, collect, and investigate in a hands-on manner that is much harder for larger organisms or more abstract topics. Many of the National Standards for Science, Mathematics, Language Arts, and Social Studies can be addressed using forestry and natural-resources lesson plans and units. Forest health topics are a great way to cover content standards, use hands-on teaching techniques, and educate students about insect and disease ecology in their own backyards. The objective of this field investigation is for students to become familiar with forest health concepts. It is targeted for middle or high school students, though younger students could do a shorter version, and older students may delve further.
The definition of a healthy forest depends on your perspective (Kolb et al., 1994). Timber managers may want to grow expensive trees and limit mortality. Wildlife sanctuaries may want to have as diverse a forest as possible for different animal species. Urban park managers may place priority on aesthetics and remove trees with decay or dead branches in the crown. Generally, the definition of a healthy forest, from an ecosystem perspective, would be one free from distress symptoms (Haskell et al., 1992). Forest distress symptoms include widespread, damaging insects, diseases, or other organisms that reduce productivity or biodiversity, disturb nutrient cycling, or reverse natural succession (Haskell et al., 1992).
Exotic or nonnative insects and pathogens are usually a problem in any forest. Exotic organisms invade native ecosystems and can permanently alter them or outcompete and eliminate native species. For example, American chestnuts once covered almost 50% of eastern North American forests (Worrall, 2012). In the early 1900s, a fungal disease was accidentally introduced from Asia, and by the 1940s, American chestnuts were tragically decimated, removing >3.5 billion trees from our forests. More recent examples could include the emerald ash borer, Asian longhorned beetle, hemlock wooly adelgid, Dutch elm disease, or beech bark disease. Exotic plants and earthworms also top the list as “ecosystem engineers” that dramatically affect forest health and biological processes.
Abiotic stress must be considered when investigating forest health. Air pollution, climate change, forest fragmentation by roads and urban development, and insufficient fire cycles can all stress trees and forests. Human actions are linked to unhealthy forests, either directly or indirectly. Increasing awareness of our influence on these problems can help us garner the knowledge to change behavior and solve issues. By continually interesting and educating students, forest managers, extension officials, policymakers, the general public, and others, we will be able to keep our forests healthy.
Forest researchers, foresters, entomologists, pathologists, and other professionals complete tree or forest surveys to identify symptoms of healthy and unhealthy forests. Visual symptoms can include insect herbivory, wounds, scars or galleries on trees, leaf deformities or galls, mushrooms or fungal hyphae (decay), and dead or dying foliage or branches (Figure 1). The following procedure is for students to compare and contrast the health condition of two separate areas of forest or woods. Students can use the provided symptom guides (Figures 2, 3, and 4) or make their own. Insect galleries and damage to tree boles or stems can be found all year round and may be easier to detect during leaf-off times. Student surveys that include foliage damage and galls would be best in late summer to leaf fall. With limited or no equipment, students can perform their forest survey at a school forest, at a local park, or on street trees (see “Extensions”).
Materials & Equipment
Measuring tape, preferably at least 10 m
Flexible measuring tape (1 to 2 m) or a “dbh” (diameter at breast height) tape that reads diameter, rather than circumference, to measure around trees. Premeasured string would work to determine what predetermined size categories a tree is in (decided by teacher).
Visual symptom guides or worksheets
This lesson has the most impact if students are able to compare the health of two areas or two forest types. A good example would be a school forest and a park or recreational area. Street trees can be used as well. Contact local extension agencies, parks, or foresters for help in identifying local areas. Students should do all measurements and visual surveys.
Depending on the size of the class, break it into pairs or groups and hand out equipment. Briefly review how to use the compass (students need to find north to start).
Assign students a plot center. Starting at 0°N, students will count and examine each tree within a certain radius of a center person holding the tape (Figure 5). A typical radial plot size for surveys in hardwoods is 11.3 m (37.2 feet radius, 0.04 ha, or 1/10th of an acre). Reduce the radius size for younger students or to take up less time in a more dense forest. It is best if plots do not overlap: students may use crayons or chalk to mark trees they have surveyed, and classroom data should not count the same symptoms on the same trees.
Each tree will be measured to compare the health of larger and smaller or older and younger trees. If you are measuring circumference, divide the answer by π (3.14159) to obtain the correct value. Approximately 1.37 m (4.5 feet) above the ground, wrap the measuring tape around the tree trunk perpendicular to the tree. Have students discuss whether this works for every tree or not and whether they should include dead trees. Depending on the trees you are working with and your time limit, assign a minimum dbh (2.5 or 10 cm). The smaller you go, the more trees will be surveyed.
Keep a worksheet of each tree’s number, size, and identified visual symptoms of tree health, essentially counting every time one type of symptom is present (i.e., not counting every leaf with spindle galls, just confirming that they are present on that tree). Depending on location and species, some symptoms will be more common than others; students could focus on those issues. Many trees will have more than one symptom; all can be recorded, or have students choose the factor they think is most stressful to the tree. Both deciduous and coniferous trees can be surveyed. The Forest Health Detective Survey Guide, Gall and Mine Guide, and Gallery Guide (Figures 2, 3, and 4) can be used as visual aids to identify symptoms. Have students sketch any unknown symptoms for later identification.
Have students analyze their own survey plot data or combine the class data together (see “Data Analysis”). Individually or as a group, determine whether the forests/trees you evaluated were healthy or unhealthy. Professionals in the field have to make this decision, determining whether the symptoms together are significantly reducing tree growth or leading to conditions that increase tree mortality. Remind students that biodiversity and decay can be beneficial for other organisms, but trees can be considered unhealthy if owners are losing timber value, aesthetics, or ecosystem protection (e.g., insect outbreaks killing trees, reducing wildlife habitat, increasing intense fire risk).
Surveying trees in one 11-m plot takes 30–90 minutes, depending on the number of trees and symptoms that students record. Summarization calculations and data analysis take 20–30 minutes if students have used Excel. Teachers should scout out the areas ahead of time to be familiar with the location and symptoms that students will encounter.
Suggested Class Schedule
Because this is primarily a lesson built on student experience outside, scheduling depends on availability. Two separate parks or forest areas could be visited in two 50-minute sessions, with a third day for analysis and discussion. Alternatively, one afternoon of field trips would allow for all surveys and analysis to be completed.
Have students calculate the percentage of healthy trees and those with stress symptoms using their survey and determine whether larger or smaller trees are damaged or vigorous. A report summarizing the symptoms found, hypothesizing causes, and making suggestions for forest health improvement could be turned in to the forest or park manager or the school principal. Students will have to consider whether symptoms are severe and damaging overall health or are of minor importance, and they may have to do some web searching if they are unsure (e.g., vandalism at a park should be addressed while leaf miners are damaging only a few leaves per tree). Data can be entered into spreadsheet software to analyze graphs (Figure 6) and use statistics where appropriate. Suggested analyses include the following:
Number of trees with and without symptoms in the two areas visited (Figure 6A). Is one “healthier” than the other? Scale up the size of your plots to represent the number of trees per area (i.e. number of trees in 0.04-ha plot × 25 = number of trees per hectare). Use mapping websites to determine how large your forest is. How many trees in the forest have symptoms?
Number of trees greater than and number less than 10 cm dbh, with and without symptoms. These represent current dominant trees (>10 cm) and the health of the future potential trees (<10 cm).
Plot the size of trees per number of cankers, rotten spots, or insect galleries (whichever symptom you see most often; Figure 6B). Some insects or pathogens attack only certain sizes of trees or branches. For example, a forester can target large trees with many cankers in the next harvest to reduce the risk of smaller trees becoming infected as they grow.
Percentage of trees with unhealthy symptoms that appear to be natural or caused by humans (Figure 6C). Insect and disease damage is usually natural unless induced by anthropogenic causes. Residual logging damage, trees too close to roads, or vandalized trees are examples of human causes.
Percentage of each visual damage symptom identified overall or by each forested area visited (Figure 6D). Are insect galleries more common, or decay fungi? Are there a lot of trees with dead and dying branches? Is vandalism a problem in urban areas?
Having students compile the data into a class report and see that it goes to someone capable of making decisions about tree health and management supports the message that trees and forests can be taken care of, similar to us when we are unhealthy. Healthy trees should be able to meet the goals of the owners or managers, such as growing timber, providing shade in a yard, or serving as valuable wildlife habitat.
Forest health is an important topic that can be linked to biology, environmental science, entomology, botany, or agriculture concepts as well as social studies or current-issues classes. The basic concepts and unanswered questions open the way for discussions and group or individual research. Rural or urban, private or public, naturally occurring or brought on by people, tree health is important to us all.
The Forest Health Detective lesson can be easily adapted for elementary grades. Provide magnifying glasses for students to examine evidence and symptoms. Circular plots could be smaller, or premeasure a square plot and use flagging to mark the outside edges. Younger children may have an easier time focusing on one symptom or organism to investigate. For example, look in an urban recreational area for vandalism or damage from mowers on trees, or in a forest with an emerald ash borer infestation to search for the indicative galleries, exit holes, and sprouts at the base of trees. Instead of software analysis, teachers may chart or graph class data for younger students on a whiteboard or overhead. A survey could also be done along transect lines, whereby a person walks a line in a forest and stops every 10 feet, measuring the nearest tree. Students may also want to do a separate survey for saplings and regeneration smaller than your minimum dbh. To do an urban street tree survey, have students examine trees between the sidewalk and street for a set amount of blocks.
Older students can go into more detailed diagnoses, research causes, and propose management solutions. Most insect and disease problems have accessible information on the web. They can identify trees to species or families (i.e., maples, oaks, birch, and pine). A professional, detailed forest health survey would also include the herbaceous (nonwoody) and exotic species present. There are excellent online sources for ID in general or for specific regions (i.e., http://www.forestryimages.org, http://uptreeid.com, http://texastreeid.tamu.edu). Have the class put on a presentation, for example to the park manager, landowner, or school principal, to encourage management for forest health. Older students can take digital pictures or illustrate insects, symptoms, or fungi and put together a forest health guide or walking-tour pamphlet of forest diversity. Make copies of their publication available for the park, school, or landowner to use as a brochure or poster display.
Insect galleries are great for studying insect life cycles because they can include characteristic egg niches, pupal chambers, mating chambers, and exit holes, and they may increase in size as the larvae grow. Similarly, galls can be measured and linked to food webs, natural selection, and genotypic variation (Cornell Institute for Biology Teachers, 2010).
The lesson and example symptom guides included here were developed for use in eastern forests, but the lesson could easily be adapted to any local terrestrial or aquatic ecosystem. For example, in prairie or savanna environments, do a survey for exotic grasses. Health indicator species of plants and macroinvertebrates are commonly sampled in streams and lakes (U.S. Environmental Protection Agency, 2012). A survey for tree health can be done year round – even during winter, examining just trunks and branches.
Glossary for Forest Health Detectives
Canker: A disease of the bark layers that causes a well-defined sunken or swollen area.
Exotic species: An organism introduced to another area outside of its native region or country.
Forest health: The apparent condition of a forest, generally free from stress and able to perform biological and ecological processes.
Gall: Abnormal growth or swelling by a plant in response to an insect or pathogen.
Gallery: An insect hole, burrow, mine, or tunnel in wood or bark.
Hyphae: The branching, filamentous portion of a fungus, usually seen as a white or dark, matted patch under bark.
Leaf miner: A type of insect that makes galleries in foliage (usually by larvae feeding).
Mushroom: The fruiting body of a fungus that we usually see.
Symptom: Any observable change in host (tree) structure or character, such as swelling, scars, galls, wilting, yellowing leaves, etc.
Wound: Any open or scarred-over break in the bark of a tree, either induced by natural causes or inflicted through human vandalism or by equipment scrapes such as in logging or plowing.